Integrated circuits are formed on semiconductor wafers, which are then sawed into semiconductor chips. The semiconductor chips may be bonded onto package substrates. During the bonding process, the solder bumps between the semiconductor chips and the package substrates are reflowed. Conventional reflow methods include convection-type reflow and thermal compressive reflow. The convection-type reflow has relatively high throughput since multiple package substrates and the overlying dies may be bonded through the reflow at the same time. However, the convection-type reflow requires a long period of time to heat solder bumps. The resulting high thermal budget may cause significant warpage in the dies, and may possibly cause delamination between low-k dielectric layers in the dies.
The thermal compressive reflow requires a lower thermal budget than the convection-type reflow. In conventional thermal compressive bonding processes, a die is stacked on a package substrate, with the solder bumps on a surface of the die being pressed against the solder bumps on the surface of the package substrate. The bond head of the thermal compressive bonding apparatus then heats the die until the solder bumps melt. After the melting of the solder bumps, solder bumps cool down to solidify. Cooling air may be blown to the bond head, the solder bumps, and the package substrate to speed up the cooling.
In the conventional bump cooling process, depending on the direction of the cooling air, solder bumps solidify at different time. The non-uniformity in the solidification of the solder bumps results in stress to be generated by the solder bumps. For example, the solder bumps that solidify later than other solder bumps may pull surrounding parts of dielectric layers in the die. In addition, the conventional air cooling has a low efficiency.
Additional conventional bump cooling processes further include water cooling, which includes attaching a water cooling apparatus to the bond head, wherein water may flow in the cooling apparatus. The cooling apparatus encircles an upper part of the bond head, which is further attached to a lower part of the bond head. The lower part of the bond head has a dimension greater than a size of the die, and is used to contact the die. The upper part, on the other hand, has a smaller size than the die. Accordingly, during the bump cooling process, the heat coming from the solder bumps and the die is transferred to the cooling apparatus through the lower part and the upper part of the bond head. This cooling method also causes non-uniformity in the cooling of solder bumps.